Liquid crystal display devices have been well known for many years. Display modes such as twisted nematic, supertwist, and hybrid aligned nematic require polarizers to generate two or more optically different states.
Guest-host liquid crystal devices operate by reorienting an anisotropic dye (the guest) which is dissolved in a liquid crystal (“LC”) host. The orientation of anisotropy of the guest is aligned with and follows the orientation of the director of the host. Guest-host LC systems are potentially attractive, especially for reflective displays, because they offer the prospect of eliminating the expensive and optically wasteful polarizers and, in the case of plastic displays, the requirement for birefringence-free substrates. Guest-host liquid crystal devices to date have had limited commercial success, mainly because suitable material properties have not been attainable.
The key parameters of a guest host display are                Physical and chemical stability, especially photostability        Concentration of the guest—to get a sufficiently high absorption in a thin layer        Dichroic ratio—to get a good contrast, the guest must be well ordered in the host, and must have a high ratio of absorption between orthogonal directions.        Viscosity of the mixture—if this is too high, the system may switch too slowly to be of use.        